We traveled to the Big Island just about the same Oct./Nov. dates you mentioned, in 2019. The Ironman Triathlon occurs earlier in Oct., so not an issue. Stayed in Waikoloa which is located in South Kohala. Have also stayed in Kona before. The water temps diving were consistent at 82º F the 2 weeks we were in Waikoloa. Not that it would sway my decision, but the wettest (rain) month on the Big Island is September.
Diving is good in both areas, but different, IMO - saw larger sea creatures (spinner dolphins, sharks, monk seal) and volcanic topography in Kona; saw more reef type structures and smaller sea life (frog fish, dragon wrasse, and more) in Waikoloa.
Jack's Diving Locker in Kona was a very good dive op as was Blue Wilderness in Waikoloa. In Kona, you want to be sure and do the Manta night dive/snorkel - we snorkeled it as we had 2 non-divers in our group - you see the same thing either way - either mantas swimming up to the surface or mantas swimming toward the bottom.
You can find several threads about diving/snorkeling the Big Island. Below is a link to my 2019 trip review.
Spent 2 weeks at Paniolo Greens in Waikoloa Village which is located in South Kohala about 25 miles north of Kona and about 5-6 miles from the shore. Was a little worried about elevation and diving but Waikoloa Village is just under 1000'. On non-diving days we hugged the coast site-seeing and...
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